Gas generating system

ABSTRACT

The present invention generally relates to gas generant compositions for inflators of occupant restraint systems, for example. A gas generating composition  12  formed in accordance with the present invention includes a carboxyl alkyl cellulosic binder. A vehicle occupant protection system  180 , and other gas generating systems, incorporate the compositions of the present invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser. No. 60/557,473 filed on Mar. 30, 2004.

TECHNICAL FIELD

The present invention relates generally to gas generating systems, and to gas generant compositions employed in gas generator devices for automotive restraint systems, for example.

BACKGROUND OF THE INVENTION

The present invention relates to nontoxic gas generating compositions that upon combustion rapidly generate gases that are useful for inflating occupant safety restraints in motor vehicles and specifically, the invention relates to thermally stable nonazide gas generants having not only acceptable burn rates, but that also, upon combustion, exhibit a relatively high gas volume to solid particulate ratio at acceptable flame temperatures.

The evolution from azide-based gas generants to nonazide gas generants is well-documented in the prior art. The advantages of nonazide gas generant compositions in comparison with azide gas generants have been extensively described in the patent literature, for example, U.S. Pat. Nos. 4,370,181; 4,909,549; 4,948,439; 5,084,118; 5,139,588 and 5,035,757, the discussions of which are hereby incorporated by reference.

In addition to a fuel constituent, pyrotechnic nonazide gas generants contain ingredients such as oxidizers to provide the required oxygen for rapid combustion and reduce the quantity of toxic gases generated, a catalyst to promote the conversion of toxic oxides of carbon and nitrogen to innocuous gases, and a slag forming constituent to cause the solid and liquid products formed during and immediately after combustion to agglomerate into filterable clinker-like particulates. Other optional additives, such as burning rate enhancers or ballistic modifiers and ignition aids, are used to control the ignitability and combustion properties of the gas generant.

One of the disadvantages of known nonazide gas generant compositions is the amount and physical nature of the solid residues formed during combustion. When employed in a vehicle occupant protection system, the solids produced as a result of combustion must be filtered and otherwise kept away from contact with the occupants of the vehicle. It is therefore highly desirable to develop compositions that produce a minimum of solid particulates while still providing adequate quantities of a nontoxic gas to inflate the safety device at a high rate.

The use of phase stabilized ammonium nitrate as an oxidizer, for example, is desirable because it generates abundant nontoxic gases and minimal solids upon combustion. To be useful, however, gas generants for automotive applications must be thermally stable when aged for 400 hours or more at 107.degree. C. The compositions must also retain structural integrity when cycled between −40.degree. C. and 107.degree. C. Further, gas generant compositions incorporating phase stabilized or pure ammonium nitrate sometimes exhibit poor thermal stability, and produce unacceptably high levels of toxic gases, CO and NO_(x) for example, depending on the composition of the associated additives such as plasticizers and binders.

Even so, the addition of additives such as binders is often necessary to retain the shape of the propellant or gas generant tablets, and inhibit fragmentation of the same over time. Certain water soluble binders, such as carboxyl cellulosic binders, exhibit hygroscopic properties given their water solubility. Accordingly, these types of binders result in compositions that often have poor thermal stability, and in particular with compositions containing preferred oxidizers such as phase stabilized ammonium nitrate.

Known water insoluble binders such as cellulosic esters or alkyl celluloses such as cellulose acetate, cellulose acetate propionate, and cellulose acetate butyrate contain a cellulose backbone substituted with only alkyl substitutions (acetyl, propionyl, butyryl) and hydroxyl groups. As a result, the compositions containing the alkyl celluloses are nonhygroscopic and also exhibit excellent thermal stability. Nevertheless, these compositions typically have poor oxygen balance thereby inhibiting the addition of energetic fuels. Furthermore, the use of organic solvents when manufacturing compositions containing these binders complicates production from a safety standard due to the flammability and volatility of solvents such as ethyl acetate, propyl acetate, and butyl acetate. Yet another concern relative to the use of binders requiring a solvent-based process is that the organic solvent remnant must be disposed of with the attendant environmental concerns.

Accordingly, ongoing efforts in the design of automotive gas generating systems, for example, include initiatives that desirably produce more gas and less solids without the drawbacks mentioned above.

SUMMARY OF THE INVENTION

The above-referenced concerns are resolved by gas generating systems including a gas generant composition containing a carboxy alkyl cellulosic binder such as carboxymethylcellulose acetate butyrate. Stated another way, compositions of the present invention contain a primary binder containing both carboxyl substitutions and alkyl substitutions. Known fuels, oxidizers, and other additives may be incorporated into these compositions as known in the art and as determined by design criteria. In accordance with the present invention, gas generating systems such as airbag inflators and vehicle occupant protection systems incorporate these gas generating compositions.

In sum, the present invention includes gas generant compositions that optimize the production of gas combustion products and minimize solid combustion products while retaining other design requirements such as reduced hygroscopicity, thermal stability, production safety, and reduced environmental impact. These and other advantages will be apparent upon a review of the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary airbag inflator containing a gas generant composition formed in accordance with the present invention.

FIG. 2 is a schematic representation of an exemplary vehicle occupant restraint system incorporating the inflator of FIG. 1 and a gas generant in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention includes gas generant compositions that contain a fuel, an oxidizer, and a primary binder. The primary binder is selected from the group of cellulosic polymers wherein each polymeric binder contains carboxyl functionality, alkyl functionality, and hydroxyl functionality. Stated another way, A preferred binder selected from this group is carboxymethylcellulose acetate butyrate (CMCAB). Other carboxy alkyl celluloses binders are contemplated, and are exemplified by those compounds containing a cellulose backbone substituted with both carboxyl substitutions and alkyl substitutions, and salts of these compounds including nonmetal, metal, and alkali and alkaline earth metal salts including potassium, sodium, strontium, and ammonium salts thereof. These salts may be formed simply be reacting the carboxy alkyl celluloses with a base such as potassium hydroxide or ammonium hydroxide.

Carboxyl substitutions include carboxymethyl, succinyl, and maleyl groups. Alkyl substitutions include acetyl, propionyl, butyryl groups with hydroxyl groups. The primary binder is generally provided at about 0.1-20%, and more preferably at 1-10% by weight of the composition.

It has been found that cellulosic polymers containing hydroxyl (—OH) groups on an anhydroglucose backbone can be reacted with various cyclic anhydrides to produce other carboxyl alkyl cellulose binders. Examples of cyclic anhydrides suitable for reaction with the base polymer include succinic anhydride and maleic anhydride. Other known anhydrides are available that will provide carboxyl functionality by the same process.

Gas generant compositions of the present invention may also contain the following constituents in the weight percents indicated. A primary fuel is selected from the group containing azoles such as 5-aminotetrazole; nonmetal salts of azoles such as potassium 5-aminotetrazole; nonmetal salts of azoles such as mono- or diammonium salt of 5,5′-bis-1H-tetrazole; nitrate salts of azoles such as 5-aminotetrazole nitrate; nitramine derivatives of azoles such as 5-nitraminotetrazole; metal salts of nitramine derivatives of azoles such as dipotassium 5-nitraminotetrazole; metal salts of nitramine derivatives of azoles such as dipotassium 5-nitraminotetrazole; nonmetal salts of nitramine derivatives such as mono- or diammonium 5-nitraminotetrazole and; guanidines such as dicyandiamide, nitroguanidine, and guanidine nitrate; salts of guanidines such as guanidine nitrate; nitro derivatives of gaunidines such as nitroguanidine; azoamides such as azodicarbonamide; nitrate salts of azoamides such as azodicarbonamidine dinitrate; and mixtures thereof. The primary fuel is typically employed at 0-80% by weight and more preferably at about 10-70% by weight. It will be appreciated that in certain compositions, the amount of binder employed will also provide fuel effective amounts of the binder whereby the binder functions as a binder/fuel. IS THIS TRUE? Accordingly, in that instance, the primary fuel may not be included in the composition. An optional secondary fuel selected from the same group of fuels is typically provided at about 0-50%, and more preferably at about 0-30% by weight.

A nonmetal or metal primary oxidizer may be selected from nitrate salts such as ammonium nitrate, phase stabilized ammonium nitrate stabilized in a known manner and more preferably with about 10% by weight of potassium nitrate, potassium nitrate, and strontium nitrate; nitrite salts such as potassium nitrite; chlorate salts such as potassium chlorate; perchlorate salts such as ammonium perchlorate and potassium perchlorate; oxides such as iron oxide and copper oxide; basic nitrate salts such as basic copper nitrate and basic iron nitrate; and mixtures thereof. The primary oxidizer may be provided at about 0.1-80% by weight, and more preferably at about 10-70% by weight. Secondary oxidizers may also be employed and are selected from the oxidizers described above. The secondary oxidizers are typically provided at about 0-50%, and more preferably 0-30%, by weight of the gas generant composition.

An optional secondary binder may be selected from cellulose derivatives such as cellulose acetate, cellulose acetate butyrate, carboxymethylcellulose, salts of carboxymethylcellulose; silicone; polyalkene carbonates such as polypropylene carbonate and polyethylene carbonate; and mixtures thereof. When employed, secondary binders may be provided at about 0-10%, and more preferably, 0-5% by weight.

An optional slag former may be selected from silicon compounds such as elemental silicon and silicon dioxide; silicones such as polydimethylsiloxane; silicates such as potassium silicates; natural minerals such as clays, talcs, and micas; fumed metal oxides such as fumed silica and fumed alumina. When employed, slag formers may be provided at about 0-10%, and more preferably, 0-5% by weight.

Other exemplary fuels, oxidizers, and other gas generant constituents are described in U.S. Pat. Nos. 5,035,757, 5,756,929, 5,872,329, 6,074,502, 6,287,400, 6,210,505, and 6,306,232, each herein incorporated by reference in its entirety.

Carboxymethylcellulose acetate butyrate (CMCAB) is available from Eastman Chemical Company in Kingsport, Tennessee. It is known as a water-dispersable polymer, which means that it can be solvated by combining a lesser amount of an organic solvent with water. Other gas generant constituents may be provided by known suppliers such as Aldrich Chemical Company.

In an aqueous/organic solvent mixture the following constituents may be added, and homogeneously mixed as they are added, in the weight percents given. A primary binder, CMCAB, at 0.1-20% and more preferably 1-10% is added to the mixture. Secondary binders may also be added at 0-10%, and more preferably 0.1-5%, and are selected from the group including cellulose derivatives such as cellulose acetate, cellulose acetate butyrate, and carboxymethylcellulose; salts of carboxymethylcellulose; silicone; polyalkene carbonates such as polypropylene carbonate and polyethylene carbonate.

Gas generant compositions of the present invention may be formed as known in the art. Examples of typical manufacturing processes include: (1) blending and/or grinding oxidizer, fuel, binders, and other components without solvent and compacting the powdered material on a press; (2) solvating the cellulosic binder in an aqueous/organic solution, adding the desired constituents such as fuel, oxidizer, and other additives, and molding into a propellant grain. The solvent is then dried off; (3) Solvating the cellulosic binder, adding oxidizers, fuels, and other components and extruding the propellant under pressure through a die to form various shapes. The shapes may then be cut to length and the solvent evaporated or heated off.

The drying process may be accelerated by applying heat to the final homogeneous mixture. Or, depending on design criteria, the drying process may be prolonged in the absence of heat, for example. Other formulation methods are contemplated including other known wet and dry mixing and compacting methods. It is contemplated that the present compositions be employed in gas generating systems. An exemplary gas generating system includes an airbag device or vehicle occupant protection system shown in FIG. 2 to include airbag modules, airbag inflators or gas generators, and more generally, vehicle occupant restraint systems, all built or designed as well known in the art.

To illustrate the processing benefit of using carboxy alkyl celluloses in gas generants, mixtures of Phase-stabilized Ammonium Nitrate (PSAN), bis tetrazole diammonium salt (BTzA), and Eastman products were prepared in the weight percents shown in Table 1. The materials chosen for comparison were Eastman Cellulose acetate butyrate CAB381-20BP and Eastman Carboxymethyl cellulose acetate butyrate CMCAB641-0.2. The compositions and burn rates are shown in Table 1. TABLE 1 Rate of burn @ 1000 Rate of burn @ 3000 Composition psi psi PSAN-82 0.16 0.40 BTzA-10 CMCAB641-0.2-8 PSAN-82 0.20 0.45 BTzA-10 CAB381-20BP-8

As shown in Table 1, the two Eastman cellulosic binders provide very similar performance when mixed in the same ratios. One benefit of using Carboxymethylcellulose Acetate Butyrate (CMCAB) in these formulations is reflected in the amount of organic solvent required for processing. Cellulose Acetate Butyrate (CAB) can only be solvated by purely organic solvents/mixtures. CMCAB can be solvated by water-organic mixtures and used in the same process, thereby decreasing the amount of environmentally harmful waste generated. Dispersion method II, as described in Eastman on-line publication gn431 allows for the use of approximately 50/50 water-organic mixtures used as solvent.

To illustrate the benefit of the addition of carboxyl groups on the polymer backbone, several gas generating compositions were formulated using varying degrees of carboxymethyl substitution and oxygen balancing to CO₂ (as is ideal for automotive applications). Phase-stabilized Ammonium Nitrate was chosen as oxidizer and bis tetrazole diammonium salt as fuel. The oxygen balance of the cellulosics and the degree of substitution (DS) of carboxymethyl (CM) groups on the cellulose backbone are shown in Table 2. A degree of substitution of 3 represents 100% substitution. All ratios are on a mass basis and formulations were based on using 10% cellulosic binder. TABLE 2 Formulation as Oxygen Cellulosic binder Oxygen Balance Balanced to CO2 Eastman CAB381-20BP −162 PSAN-86.3 (CM DS = 0) BTzA-3.7 CAB-10.0 Eastman CMCAB641-0.2 −158 PSAN-85.9 (CM DS = 0.35) BTzA-4.1 CMCAB-10.0 Modified CMCAB-01 −139 PSAN-84.0 (CM DS = 1.0) BTzA-6.0 CMCAB-10.0 CMC*-03 −100 PSAN-79.9 (CM DS = 3.0) BTzA-10.1 CMC-10.0 (*carboxymethylcellulose (CMC))

With the addition of energetic fuels, such as bis-tetrazole diammonium salt, ballistic performance is enhanced. The above table displays the added benefit of addition of carboxy groups on the polymer backbone. It is possible to improve the oxygen balance even further by forming salts of CMCAB and other carboxy alkyl cellulose polymers, such as sodium, potassium, strontium or ammonium salts.

One advantage of celluloses like CMCAB over celluloses like CMC is that the hygroscopicity is substantially reduced due to a relative decrease in the polar nature of CMCAB as compared to CMC by the alkyl substitutions. The hygroscopic properties of CMC and its salts make processing very difficult given that propellants used in automotive passenger restraints, for example, must pass extreme condition tests such as thermal shock and high-temperature heart aging. Furthermore, gas generant compositions used in automotive applications must also exhibit a useful lifetime of many years in order to meet customer requirements. Poor heat-temperature aging indicates a short shelf-life of gas generant compositions associated therewith. The introduction of moisture into these materials may cause disadvantages such as a potential decrease in performance, decomposition during heat aging, and structural breakdown of propellant grains during aging and thermal shock. Also, the thermal stability of carboxy celluloses like CMC with preferred oxidizers such as ammonium nitrate or phase stabilized ammonium nitrate is significantly reduced due to interactions with the relatively greater number of hydroxyl substitutions on the polymer backbone (as compared to CMCAB). In aging mixtures of CMC with ammonium nitrate at 107C, CMC and its salts exhibit a color change from off-white to black, and mass loss of 0.7% to 10% in 400 hours, while mixtures with alkyl celluloses, such as CAB with a relatively lower hydroxyl substitution, exhibit mass loss of less than 0.2%, even after 1000 hours at 107C. This fact reflects favorably on the alkyl substitutions or groups in CMCAB.

Accordingly, exemplary advantages of celluloses like CMCAB over celluloses such as Cellulose Acetate (CA), Cellulose Acetate Propionate (CAP), and Cellulose Acetate Butyrate (CAB) at least one or more of the following advantages: improved oxygen balance (as illustrated in Table 2), and the capacity to be solvated with a mixture of organic solvent and water to reduce the amount of harmful waste in processing, and to provide a safer process with regard to flammability and volatility.

As shown in FIG. 1, an exemplary inflator incorporates a dual chamber design to tailor the force of deployment an associated airbag. In general, an inflator containing a gas generant 12 formed as described herein may be manufactured as known in the art. U.S. Pat. Nos. 6,422,601, 6,805,377, 6,659,500, 6,749,219, and 6,752,421 exemplify typical airbag inflator designs and are each incorporated herein by reference in their entirety.

Referring now to FIG. 2, the exemplary inflator 10 described above may also be incorporated into an airbag system 200. Airbag system 200 includes at least one airbag 202 and an inflator 10 containing a gas generant composition 12 in accordance with the present invention, coupled to airbag 202 so as to enable fluid communication with an interior of the airbag. Airbag system 200 may also include (or be in communication with) a crash event sensor 210. Crash event sensor 210 includes a known crash sensor algorithm that signals actuation of airbag system 200 via, for example, activation of airbag inflator 10 in the event of a collision.

Referring again to FIG. 2, airbag system 200 may also be incorporated into a broader, more comprehensive vehicle occupant restraint system 180 including additional elements such as a safety belt assembly 150. FIG. 2 shows a schematic diagram of one exemplary embodiment of such a restraint system. Safety belt assembly 150 includes a safety belt housing 152 and a safety belt 100 extending from housing 152. A safety belt retractor mechanism 154 (for example, a spring-loaded mechanism) may be coupled to an end portion of the belt. In addition, a safety belt pretensioner 156 containing propellant 12 may be coupled to belt retractor mechanism 154 to actuate the retractor mechanism in the event of a collision. Typical seat belt retractor mechanisms which may be used in conjunction with the safety belt embodiments of the present invention are described in U.S. Pat. Nos. 5,743,480, 5,553,803, 5,667,161, 5,451,008, 4,558,832 and 4,597,546, incorporated herein by reference. Illustrative examples of typical pretensioners with which the safety belt embodiments of the present invention may be combined are described in U.S. Pat. Nos. 6,505,790 and 6,419,177, incorporated herein by reference.

Safety belt assembly 150 may also include (or be in communication with) a crash event sensor 158 (for example, an inertia sensor or an accelerometer) including a known crash sensor algorithm that signals actuation of belt pretensioner 156 via, for example, activation of a pyrotechnic igniter (not shown) incorporated into the pretensioner. U.S. Pat. Nos. 6,505,790 and 6,419,177, previously incorporated herein by reference, provide illustrative examples of pretensioners actuated in such a manner.

It should be appreciated that safety belt assembly 150, airbag system 200, and more broadly, vehicle occupant protection system 180 exemplify but do not limit gas generating systems contemplated in accordance with the present invention.

It will be understood that the foregoing descriptions of various embodiments of the present invention are for illustrative purposes only, and should not be construed to limit the breadth of the present invention in any way. As such, the various structural and operational features disclosed herein are susceptible to a number of modifications, none of which departs from the scope of the present invention as defined in the appended claims. 

1. A gas generating system comprising: a gas generating composition containing a binder selected from the group consisting of carboxy alkyl cellulose polymers and salts thereof, said polymers containing carboxyl, alkyl, and hydroxyl functionality.
 2. The gas generating system of claim 1 wherein said gas generating composition further comprises a fuel and an oxidizer.
 3. The gas generating system of claim 1 wherein said binder is carboxymethylcellulose acetate butyrate.
 4. The gas generating system of claim 1 wherein said gas generating system is a vehicle occupant protection system.
 5. A gas generating system comprising: a gas generating composition comprising a fuel, an oxidizer, and carboxymethylcellulose acetate butyrate.
 6. A gas generating composition comprising: a fuel; an oxidizer; and a cellulosic binder selected from the group consisting of cellulosic polymers containing alkyl and carboxyl functionality, and salts thereof.
 7. The gas generating composition of claim 6 wherein said binder is carboxymethylcellulose acetate butyrate. 